1. Field of the Invention
The present invention relates generally to a method of creating filters which may be used to select and/or reject pre-determined frequencies of electromagnetic radiation. The invention relates more particularly to a method of making solid crystalline materials in which colloidal electrically charged particles form an ordered dispersion and are solidified into a hardened material which has certain predetermined filtering characteristics.
In another embodiment of this present invention, submicron periodic materials are employed for size selective particle and molecular filtration based in part on the self-assembly properties of crystalline colloidal arrays. In this embodiment, the material may be passive or active and have switchable filtration properties.
2. Description of the Prior Art
It has been recognized that colloidal dispersions of polymer particles in various solvents can form crystalline structures having lattice spacings comparable to the wavelength of ultraviolet, visible and infrared radiation. Bragg diffraction techniques have been used to examine these polymer colloidal crystals with a view towards identifying their interparticle spacing, lattice parameters and phase transitions. U.S. Pat. No. 4,627,689 discloses a crystalline narrow band radiation filter which is made by forming a highly ordered crystalline colloidal structure within a cell. The crystalline colloidal structure is formed by dispersing electrically charged particles, for example, polystyrene particles within an appropriate solvent. U.S. Pat. No. 4,632,517 also discloses a narrow wavelength band filtering device created by forming a highly ordered crystalline colloidal structure within a cell.
More recently, it has been known that these crystalline structures can be very useful and that such structures have many practical applications for filter devices. In many instances it is necessary or desirable to filter out narrow bands of selected wavelengths from a broader spectrum of incident radiation while permitting the transmission of adjacent wavelengths. High spectral-purity commercial monochromators which are available for this purpose generally use a plurality of gratings and prisms. However, such devices are extremely complex, bulky and expensive. U.S. Pat. No. 4,632,517 discloses another crystalline colloidal narrow band radiation filter which may involve polystyrene particles. The device of this patent forms the basis for a mechanically simple and highly efficient monochromator. It has application in improved systems for investigating Raman or emission spectra of selected sample materials. U.S. Pat. No. 4,632,517 disclosed a type of solid structure in that with a lattice spacing gradient being formed and as part of this process a "freezing" of certain conditions is achieved using polymerization techniques. However, this suggestion did not disclose the unique aspects of the method and product of the present invention for forming filtering devices which are entirely solid and self-supporting.
The disclosures of U.S. Pat. Nos. 4,627,689 and 4,632,517, are expressly incorporated herein by reference.
Other filtering devices have also been known. See, for example, U.S. Pat. No. 4,803,688 which discloses ordered colloidal suspension optical devices. This patent relates to the addition of a water based polymer to a colloidal structure.
A nonlinear optical device which has a high speed switching capability at high radiation intensities and which can be used for rejecting certain undesired wavelength bands from such high intensity radiation is disclosed in Asher U.S. patent application Ser. No. 07/999,487 filed Dec. 30, 1992, entitled "A Method of Making An Optically Nonlinear Switched Optical Device and Related Devices." This application is owned by the assignee of the present application. This application discloses a method for making a nonlinear optical device and a related optical device. The filter effectively resists transmission of about 99.9% of radiation from a wavelength band. The material may operate as a high speed optical switch in that it becomes opaque to radiation within several nanoseconds.
Although an allusion was made to solid devices in some of the above-mentioned prior art, these patents involve crystalline colloidal structures which are not solids and are not self-supporting. Because of their high peak absorbance value, state-of-the-art colloidal crystalline array filters may be widely used for eye protection and sensor protection. However, a more rugged filter would obviously have wider application. As a result, there has been a need for a solid filter. Solids provide better mechanical stability and machinability. Non-solids, on the other hand, are subject to becoming disordered upon vibration and shock. In addition, liquid media can undergo phase transitions quite easily by freezing or boiling and this may often be undesirable.
For these reasons a solid structure is more desirable in many applications. For example, solid filtering devices are often necessary for filtering out certain bands of radiation in aviation and space travel, as they provide mechanical rigidity and this allows for a greater range of design features. It has heretofore been an extremely difficult polymer chemistry problem to create such a solid filter. See generally U.S. Pat. No. 5,131,736.
Despite all of these optical radiation filtering devices, there still remains a need for a porous polymerized film adapted to be employed for the filtration of solid or liquid materials of relatively small size. Such a filter can be of fixed or adjustable porosity. There also remains a need for the development of filters from submicron periodic materials that are polymerized into porous gel membrane filters and can be used for size selective submicron particle and molecular filtration. There also remains a need to provide a simple method of creating a gel membrane filter whose interstices allow separation of predetermined size submicron material which may be a solid, solids in a liquid, liquid out of liquids or a virus in a liquid.